Kinetics and mechanism of the oxidation of disaccharides by CrVI

Abstract

The oxidation of D-lactose, D-maltose, D-melibiose, and D-cellobiose by CrVI yields the corresponding aldobionic acid and Cr3+ as final products when an excess of reducing disaccharide over CrVI is used. The rate law for the CrVI oxidation reaction is expressed by –d[CrVI]/dt = kH [disaccharide][CrVI], where the second-order kinetic constant, kH, depends on [H+]. The relative reactivity of the disaccharides with CrVI is expressed as follows: Mel > Lac > Cel > Mal, at 33°C. In acid medium, intermediate CrV forms and reacts with the substrate faster than CrVI. The EPR spectra show that five- and six-coordinate oxo-CrV intermediates are formed, with the disaccharide acting as bidentate ligand. Five-coordinate oxo-CrV species are present at any [H+], whereas six-coordinate ones are observed only at pH < 2, where they rapidly decompose to the redox products. In the pH 3–7 range, where hexa-coordinate oxo-CrV species are not observed, CrV complexes are stable enough to remain in solution from several days to several months.Key words: chromium, saccharides, kinetics, EPR.